1. Field of the Invention
The present invention relates generally to the field of magnetic disk drives, and more particularly to an adhesive that is specially adapted to attaching a read/write head to a suspension of a head gimbal assembly within a magnetic disk drive.
2. Description of the Prior Art
Magnetic disk drives are used to store and retrieve data for digital electronic apparatuses such as computers. In FIGS. 1 and 2, a magnetic disk data storage system 10 includes a sealed enclosure 12, a disk drive motor 14, and a magnetic disk, or media, 16 supported for rotation by a drive spindle 17 of motor 14. Also included are an actuator 18 and an arm 20 attached to an actuator spindle 21 of actuator 18. A suspension 22 is coupled at one end to the arm 20 and at another end to a read/write head 24. The suspension 22 and the read/write head 24 are commonly collectively referred to as a head gimbal assembly (HGA). The read/write head 24 typically includes an inductive write element and a magnetoresistive read element that are held in a very close proximity to the magnetic disk 16. As the motor 14 rotates the magnetic disk 16, as indicated by the arrow R, an air bearing is formed under the read/write head 24 causing the read/write head to lift slightly off of the surface of the magnetic disk 16, or, as it is commonly termed in the art, to “fly” above the magnetic disk 16. Data bits can be written or read along a magnetic “track” of the magnetic disk 16 as the magnetic disk 16 rotates past the read/write head 24. The actuator 18 moves the read/write head 24 from one magnetic track to another by pivoting the arm 20 and the suspension 22 in an arc indicated by arrows P. The design of magnetic disk data storage system 10 is well known to those skilled in the art.
The magnetic disk data storage industry has been very, successful at deriving ever greater data densities on magnetic disks 16 by pursuing the miniaturization of various components such as the read/write head 24. Along with continued miniaturization has also come ever increasing design tolerances. With particular reference to mounting the read/write head 24 to the suspension 22, miniaturization has created several challenges. First, there is a need to reliably mount the read/write head 24 to the suspension 22 within very strict positional tolerances so that in operation the read/write head 24 flies within a very narrow tolerance range around the optimum height and angle. Second, there is a need to rapidly tack the read/write head 24 to the suspension 22 to reduce processing time and lessen manufacturing costs. Third, there is a need to create an electrical connection between the read/write head 24 and the suspension 22 to dissipate static charges that build up on the read/write head 24 during operation. This third requirement has become increasingly important as read/write heads 24 have been further miniaturized because as the sizes of the read and write elements have become smaller they have also become increasingly susceptible to damage from electrostatic discharge (ESD). Thus, the acceptable voltage difference between the read/write head 24 and the suspension 22 continues to decrease as the size of the read/write head 24 shrinks.
Bonding the read/write head 24 to the suspension 22 is commonly accomplished with adhesives. FIG. 3 illustrates a bonding process that utilizes two adhesives, 30 and 32, placed at opposite ends of the read/write head 24. A drop of a quick-setting adhesive 30 is used to rapidly tack the read/write head 24 to the suspension 22, while a drop of a conductive adhesive 32 is used to provide the electrical connection between the read/write head 24 and the suspension 22. The adhesives 30 and 32 presently in use are not compatible with one another and cannot be allowed to mix, so they must be placed at a sufficient distance from one another.
With respect to the conductive adhesive 32, a variety of alternatives have been tested. The most common technique for creating a conductive adhesive is to load a binder with a conductive filler. The benchmark in the electronics industry is epoxy filled with flakes of silver, commonly referred to as silver epoxy. Silver epoxy requires high curing temperatures in excess of 80° C. and does not cure rapidly, taking in excess of 10 minutes to fully cure. The lengthy curing time makes silver epoxy poorly suited to high throughput automated manufacturing processes.
The quick-setting adhesive 30 includes a ultraviolet (UV) and/or visible photoinitiator and a thermal initiator. When the quick-setting adhesive 30 is exposed to UV illumination the UV photoinitiator causes the quick-setting adhesive 30 to rapidly form an initial bond between the read/write head 24 and the suspension 22. As can be seen in FIG. 3, due to the geometry of the parts, UV illumination can only be directed to the quick-setting adhesive 30 from the sides, and from the bottom through specially placed UV holes, restricting the amount of illumination that can reach the quick-setting adhesive 30. Accordingly, a short exposure to illumination is typically not sufficient to fully cure the quick-setting adhesive 30 and much of the initial bond strength is developed around the periphery of the drop rather than through the interior.
A short initial illumination is, however, typically sufficient to tack the read/write head 24 to the suspension 22. Thereafter, the assembly is baked to activate the thermal initiator to complete the curing of the quick-setting adhesive 30 and to also cure the conductive adhesive 32. It should be noted, however, that as the two adhesives 30 and 32 cure, stresses are developed in the read/write head 24 that can cause it to warp, a process known as crowning. Although the process of using the quick-setting adhesive 30 in conjunction with the conductive adhesive 32 has been heretofore sufficient, it is not viewed as sufficient for ever smaller read/write heads 24. In particular, there is simply less space on a backside surface of a smaller read/write head 24 to place two distinct adhesive drops without allowing them to mix. Further, as read/write heads 24 become smaller and positional mounting tolerances grow tighter, crowning becomes increasingly problematic. Thus, it is desirable to find an adhesive that is both quick-curing and electrically conductive so that the manufacturing process can be reduced to a single drop delivered to the middle of the backside of the read/write head 24.
Another drawback of the present two-adhesive bonding process is that the electrical conductivity of the conductive adhesive 32 decreases as the applied voltage decreases. As read/write heads 24 are made increasingly smaller, as previously noted, lesser amounts of charge become sufficient to create ESD damage. Accordingly, good conductivity is required for smaller voltage differences between the read/write head 24 and the suspension 22. To achieve better conductivity at lower voltages, one solution has been to metallize the backside of the read/write head 24. This can be a multi-step process involving cleaning steps and multiple metal depositions. The metallization process therefore increases the time and the expense of the over-all manufacturing/bonding process.
Towards the more general goal of a UV curable and electrically conductive adhesive, U.S. Pat. No. 4,999,136 issued to Su et al. discloses a UV curable conductive resin. Su et al. discloses a variety of conductive filler materials including metal flakes, metal powders, metal coated glass beads, metal coated flakes, metal coated mica, and mixtures thereof. Although the embodiments taught by Su et al. are suitable for a myriad of applications including chip bonding, electrostatic shielding, electrical contacts, and so forth, none have been found to be well suited for the particular application of bonding a read/write head 24 to a suspension 22. For example, where the filler takes the form of a flake, too great of a percentage of the illumination is blocked and these adhesives do not develop good initial bond strengths. Reducing the loading of the flakes in the epoxy binder increases initial bond strength, but at the expense of conductivity.
On the other hand, metal coated glass beads tend to block less UV illumination, and may actually favorably forward-scatter UV illumination towards the interior of a drop, and therefore can present better initial bonding characteristics. However, metal coated glass beads present other problems in the present context. In particular, cured epoxy filled with glass beads does not retain good conductivity through thermal cycling like that encountered in the typical operating environment. It has been found that the mismatch in coefficients of thermal expansion between the glass beads and the surrounding epoxy binder causes the beads to pull away from the surfaces of the read/write head 24 and the suspension 22 at elevated temperatures leading to a loss of conductivity when it is most needed. The rigidity of the glass beads also creates very limited contact areas with the surfaces of the read/write head 24 and the suspension 22. This further limits conductivity through the cured adhesive, and makes it more likely that contacts will be lost during thermal cycling.
Another problem with metal-coated glass beads is that it is difficult to obtain tightly controlled size distributions for the average particle sizes needed for mounting read/write heads 24 to suspensions 22. FIG. 4 illustrates this problem. As can be seen in FIG. 4, one over-sized bead can create severe misalignment of the read/write head 24 with respect to the suspension 22.
Accordingly, what is needed is a UV curable and electrically conductive adhesive that can achieve a good initial bond strength from a short exposure to UV illumination and that can maintain good electrical conductivity at low applied voltages through periods of thermal cycling.